extension tube lenght alters focal lenght?
at the moment I have approximatedly 10-11 cm extension tube lenght (80mm and pushed out a bit, as well as the camera mount adapter out a bit.
one of the photos below was taken at 300mm (zuiko 75-300) the other with the 600mm celestron 80ED, and sitting at the same spot. Sensor plane difference might have been a few centimeters, give or take. While upsizing the 300mm to match the other it took 255% - giving me approx 765mm.
Am I doing something wrong here? or can the small variance of a few centimeters me holding the camera give me so much of a diff? I mean, I sat on the same spot and just switched lens.